Improve Modal docs describing iOS only support

Summary:
Adds a note that the `Modal` component is only available in iOS.

I hit a bit of a pickle this weekend after realizing a handful of components I built will have to be rewritten. Unfortunately there's no notes in the documentation that the support is limited. Hopefully this can contribute towards avoiding those situations
Closes https://github.com/facebook/react-native/pull/4592

Reviewed By: svcscm

Differential Revision: D2727634

Pulled By: androidtrunkagent

fb-gh-sync-id: 2d0efcca8e17d16cf63d592e235261cea63e59ea

Libraries/Modal/Modal.js

@@ -31,6 +31,8 @@ var RCTModalHostView = requireNativeComponent('RCTModalHostView', null);
  * Navigator instead of Modal. With a top-level Navigator, you have more control
  * over how to present the modal scene over the rest of your app by using the
  * configureScene property.
+ *
+ * This component is only available in iOS at this time.
  */
 class Modal extends React.Component {
   render(): ?ReactElement {